1. Field of the Invention
The present invention is directed to a local coil for acquiring nuclear magnetic resonance signals from an examination subject, for use in the spectroscopic analysis or the construction of an image of the subject.
2. Description of the Prior Art
A local coil for use in acquiring nuclear magnetic resonance signals from an examination subject is known from Published European application No. 0 222 982, corresponding to co-pending U.S. application Ser. No. 921,779, filed Oct. 22, 1986 (Zabel, Bader and Lorenz). This coil has a coaxial conductor member forming a first turn or winding of the coil, with the inner conductor exiting from the jacket shielding at the end of the coaxial conductor member, and being electrically connected to the jacket at the starting point of the first turn.
Local coils of this type are used in nuclear magnetic resonance tomography and nuclear magnetic resonance spectroscopy when specific portions of an examination subject are to be analyzed or imaged with an improved signal-to-noise ratio. As is known, the examination subject is disposed in a uniform fundamental field of a magnetic resonance installation. The local coil is positioned in the proximity of the region to be examined. Nuclear spins in the examination subject are excited by radio-frequency pulses in the region to be examined. This excitation can ensue either using the local coil, or using an antenna covering the entire examination region. Subsequently, the nuclear magnetic resonance signal emitted by the excited spinning nuclei is detected by the local coil, and stored in the installation. Gradient magnetic fields may be additionally superimposed on the uniform fundamental magnetic field for topically resolved spectroscopy and for imaging.
A conventional locl coil of this type for explaining the problems in this technology is shown in FIG. 1. This local coil consists of two coaxial windings or turns 1 and 2. An increase in the total inductivity of the local coil, and thus as enhancement of the signal-to-noise ratio, are achieved by the presence of the second winding, in comparison to a coil having a single winding. The local coil is connected via a coaxial line 4 to a receiver or to a combined transmission/reception stage 5. The coaxial line 4 is asymmetrical relation to ground. In order to have a connection of the local coil which is symmetrical relative to ground, a balancing circuit must be provided within a matching unit 3. The balancing circuit consists of a differential variable capacitance, consisting of two capacitors 3a and 3b connected in series. The differential variable capacitance is connected to the two terminals of the windings 1 and 2. The inner conductor of the coaxial line 4 is connected to a terminal of the local coil via a variable matching capacitor 3c, and the jacket shielding of the coaxial line is connected to the junction of the two capacitors 3a and 3b.
With this arrangement, a differential variable capacitance, i.e., two capacitors having a shared matching capability, is required. Moreover, the balancing circuit is not independent of frequency. The matching stage 3 must be disposed in the immediate proximity of the local coil, which is a practical disadvantage in positioning the local coil on the examination subject.
The aforementioned European application No. 0 222 982 discloses a local coil which can be connected to a coaxial line without the need for a balancing circuit. The local coil is formed by a loop of a coaxial conductor. To increase the total inductivity, the coaxial conductor can have a plurality of turns or windings. In this case, however, the local coil becomes unwieldy due to the relatively large diameter of the coaxial line, and capacitive detuning by the examination subject also increases due to the large conductor surface. Moreover, if the turns or windings are to be respectively disposed in individual planes, which are parallel to each other, the transition between the turns, i.e., between the planes, becomes difficult to manufacture due to the large diameter of the jacket shielding.